1. Field of the Invention
The present invention is in the field of bicycle frames, and it relates particularly to a monocoque-type bicycle frame.
2. Description of the Prior Art
The conventional bicycle frame has for many years been, and still is, what is commonly referred to as the "tube frame," which is an assembly of pipes that are welded together. The tube-type bicycle frame has several inherent disadvantages which have resulted in numerous attempts to find some other solution to a satisfactory bicycle frame. One disadvantage of the tube frame is its excessive cost to manufacture because of the numerous manufacturing steps required. Thus, all of the tubes of the frame must first be welded together, then they must be sanded, with particular attention directed to the regions of the welds; then the assembled frame must be finished, dipped for corrosion protection; sprayed with primer, and then sprayed with paint, with suitable drying time in between the dipping and spraying steps.
Another disadvantage of the tube-type bicycle frame is the substantial weight penalty that must be paid in order to achieve torsional rigidity. This is not a serious problem in ordinary street bicycles where weight is not a big factor. However, in competition bicycles such as racing bicycles, and particularly in BMX (bicycle motocross) racing bikes, strength-to-weight is a critical factor, and in this regard the most important strength factor is torsional rigidity. The weight of a bicycle frame may be reduced to only approximately 2.5 pounds for serious racing, and this can only be achieved in a tube frame with considerable sacrifice of torsional rigidity.
Another problem with the tube-type bicycle frame is that it presents an undesirably large amount of frontal area to the relative wind, and of course this is most important in racing bicycles where considerable speeds are achieved. Although the tube frame does not have the appearance of large frontal area, each tube that is not directed longitudinally toward the relative wind presents its own separate frontal area, and when these frontal areas are added up they result in a considerable amount of undesirable parasitic drag at high speeds.
A further problem with the tube-type bicycle frame is that in order to achieve the very light weights necessary for competition bicycles, exotic "space age" type metals must be employed, and these are both expensive and difficult to work.
Because of these problems with tube frames, there have been numerous attempts to replace the tube frame with a frame in the form of a hollow shell of formed or stamped sheet metal, molded fiberglass or the like. However, none of these attempts have proved completely satisfactory, and hence none of these hollow shell-type bicycle frames have come into widespread use. Although some of these attempts resulted in hollow shell bicycle frames that were simpler to manufacture than tube frames, the prior art hollow shell-type bicycle frames nevertheless had much the same problems as the tube frames, namely, they did not have an adequate strength-to-weight ratio, and in particular they did not have adequate torsional rigidity, even though they were in many instances considerably heavier than the conventional tube frame. Also, although considerable attention was given to aesthetics and curved shapes, the prior art hollow shell-type bicycle frames had undesirably large frontal areas and were generally aerodynamically unclean. Examples of prior U.S. patents disclosing hollow shell bicycle frames having these deficiencies are U.S. Pat. Nos. 2,182,828, to Stutsman et al., 2,537,325, to Bowden et al., 2,584,249, to Jaulmes, 3,233,916, to Bowden, 3,375,024, to Bowden and 4,230,332, to Porsche. Of these six prior U.S. patents, only the frame disclosed in the Porsche patent, 4,230,332, would appear to be reasonably light in weight, each wall of its hollow shell being made of an aluminum honeycomb (or balsa) core sandwiched between aluminum or fiberglass sheets. However, the configuration of this Porsche frame had opposed front and rear concavities which inherently gave the frame poor torsional rigidity relative to its weight and an aerodynamically unclean shape.
Metal stampings forming hollow shell frames for tricycles or other velocipedes are disclosed in Anderson U.S. Pat. No. 2,091,933 and Cockburn U.S. Pat. No. 4,389,055, but these frames have the same disadvantages noted above for prior art hollow shell bicycle frames. Hollow shells with the same disadvantages have been disclosed for motorscooters in U.S. Pat. Nos. 2,792,899, to Piatti and 2,908,501, to Lossau, in British Pat. No. 784,421 to Victoria-Werke. Similar hollow shell frames have been used for motorcycles, but in general these have the same disadvantages noted above for hollow shell bicycle frames, and additionally the strength requirements for motorcycles are, in general, so severe that such hollow shell frames when employed in motorcycles required additional structural reinforcement, such as use of the motor itself as a mechanical strength member, as disclosed in Asakura U.S. Pat. No. 4,334,589. Other hollow shell motorcycle frames which do not in any way solve the problems noted above in prior art bicycle hollow shell frames are U.S. Pat. Nos. 2,755,873, to Klaue, 4,171,731, to Hilber, 4,347,909, to Takemura et al. and 4,265,330 to Silk et al.
One of the things that made all of the prior art hollow shell bicycle frames undesirably bulky and heavy and of large frontal area and poor aerodynamic shape was that in each instance of which applicants are is aware the pedal sprocket or chain or other drive connection was enclosed within the hollow shell frame. This prevented a completely enclosed monocoque frame from being established, and it resulted in a frame which could not be completely filled with a rigid foam material so as to be compression loaded or prestressed to increase the overall torsional rigidity and resistance to side impacts. This defect is present in each of the to U.S. Pat. Nos. 2,182,828, to Stutsman et al., 2,537,325, to Bowden, 2,584,249, to Jaulmes, 3,233,916, to Bowden, 3,375,024, to Bowden and 4,230,332, to Porsche.
Applicant is not aware of any prior art compression loading or other form of prestressing of a hollow shell or monocoque-type bicycle frame for the purpose of increasing its strength-to-weight ratio, and in particular its torsional rigidity-to-weight ratio. U.S. Pat. No. 4,411,333 to Bothwell discloses a motorcycle fairing shell filled with a foam plastics material, but this material is a deformable or elastomeric foam adapted to deform under impact and thereby absorb some of the energy of an impact, which is a use of foam material generally opposite to that of compression loading or prestressing which requires a foam material that hardens into a rigid block.